Slip control system for a vehicle

ABSTRACT

When a slip value of the driven wheel of a vehicle on a road surface reaches a predetermined value, the braking force is applied to the driven wheel by way of slip control. The slip control by application of the braking force to the driven wheel is inhibited when the vehicle speed reaches a high speed above a predetermined vehicle speed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to slip control system for a vehicle and,more particularly, to a slip control system for the vehicle, adapted toprevent a slip of the driven wheels on a road surface from becomingexcessive by controlling torque to be applied to the driven wheels.

2. Description of Related Art

Prevention of a slip of the driven wheels from becoming excessive iseffective in order to efficiently provide a propulsive force of thevehicle as well as to provide safety in terms of prevention of a spin ofthe vehicle body and so on. In order to prevent an excessive amount of aslip of the driven wheels, torque to be applied to the driven wheels asa cause of slipping may be reduced.

Slip control of this type is disclosed, for example, in U.S. Pat. Nos.4,484,280 and 4,583,611 (corresponding to Japanese Patent UnexaminedApplication (kokai) Nos. 16,948/1983 and 56,662/1985, respectively).They disclose technique of reducing the torque applied to the drivenwheels by braking the driven wheels by brakes as well as reducing thetorque to be generated by the engine. More specifically, U.S. Pat. No.4,484,280 (corresponding to Japanese Patent Unexamined Application(kokai) No. 16,948/1983) discloses the technique of reducing the torqueto be applied to the driven wheels only by braking the driven wheelswhen a slip of the driven wheels is small and by reducing the torquegenerated by the engine, on top of braking the driven wheels, when aslip of the driven wheels becomes larger. U.S. Pat. No. 4,583,611(corresponding to Japanese Patent Unexamined Publication (kokai) No.56,662/1985) discloses the technique in which, when one of the left-handand right-hand driven wheels is larger in slippage than the other, onlythe driven wheel having a larger slip value is braked and, when both ofthe driven wheels are larger in slippage, the two driven wheels arebraked while the torque to be generated by the engine is reduced. Insummary, the technique as disclosed in the two prior patent publicationsas described hereinabove uses the braking of the driven wheel or wheelsby the brake or brakes as a main means and the reduction in the torquegenerated by the engine as an auxiliary means.

As a result of experiments, it has been found that, for a slip controlsystem of the type reducing the torque to be applied to the driven wheelby braking when a slip ratio of the driven wheel on pavement reaches avalue that is larger than a predetermined value, the vehicle becomeslikely to be unstable at the time of driving at high speed. This seemsto arise because a posture of the vehicle gets unstable due to a closecoupling of the brake with the driven wheels if the braking force wouldnot be controlled in an appropriate manner, although a response isexcellent.

SUMMARY OF THE INVENTION

Therefore, the present invention has the object to provide a slipcontrol system for a vehicle, which is adapted to ensure animplementation of the slip control using at least the brake, withoutimpeding stability of the vehicle at the time of running at high speed.

In order to achieve the object, the present invention consists of a slipcontrol system for a vehicle, as shown in a block diagram in FIG. 21,which comprises:

a braking force adjusting means for adjusting a braking force of thebrake for a driven wheel;

a slip detecting means for detecting a slip value of the driven wheel ona road surface;

a brake control means for operating the brake by controlling the brakingforce adjusting means when the slip value detected by the slip detectingmeans reaches a a predetermined value;

a vehicle speed detecting means for detecting a vehicle speed; and

an inhibition means for inhibiting operation of the brake control meansat the time of running at high speed at which the vehicle speed detectedby the vehicle speed detecting means is equal to or greater than thepredetermined value.

With the above arrangement, the slip control system according to thepresent invention prevents unstabilization of the vehicle due to brakingbecause the braking for the slip control is inhibited at the time ofrunning at high speed.

In many occasions, those who can drive the vehicle at high speed on aslippery road surface are skilled drivers, and such skilled drivers donot usually require the slip control. Further, it is preferred that theslip control by means of the engine is accepted yet the slip control bybraking is inhibited at the time of high speed running, while the slipcontrol can be implemented by adjusting torque to be generated by theengine, on top of the slip control by braking. Furthermore, it ispreferred that, for the vehicle with an automatic transmission,downshifting is inhibited at the time of high speed, when the slipcontrol is inhibited by braking. More preferably, the slip control isinhibited by braking in a high speed range in which no downshiftingoccurs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatical representation of a slip control systemaccording to an embodiment of the present invention.

FIGS. 2(a), 2(b), 2(c) and 2(d) are sectional views showing varyingstates of operation of a throttle opening angle adjusting mechanism.

FIG. 3 is a diagram showing contents of traction control.

FIG. 4A is a diagram representing one example of shift characteristicfor ordinary running.

FIG. 4B is a diagram representing one example of shift characteristicfor traction control.

FIGS. 5, 6 and 7 are maps for control to be used in accordance with thepresent invention.

FIGS. 8, 9, 10, 11, 12, 13A, 13B, 14, 15, 16, 17 and 20 are flowchartsshowing control examples in accordance with the present invention.

FIGS. 18 and 19 represent signals to be transmitted and received betweenthe control unit for traction control and the control unit for ABScontrol, respectively.

FIG. 21 is a block diagram showing an outline of the construction of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in more det ail with referenceto the accompanying drawings.

As shown in FIG. 1, an automobile A is shown to be of a rear wheel drivesystem and has a left-hand front wheel 1FL and a right-hand front wheel1FR as undriven wheels, and a left-hand rear wheel 1RL and a right-handrear wheel 1RR as driven wheels. An engine 2 loaded on a front portionof the vehicle body generates torque that is then transmitted through anautomatic transmission 3, a propeller shaft 4 and a differential gear 5to a left-hand drive shaft 6L and then to the left-hand rear wheel 1RL,on the one hand, and to a right-hand drive shaft 6R and then to theright-hand rear wheel 1RR, on the other hand.

Transmission

Referring to FIG. 1, the automatic transmission 3 comprises a torqueconverter 11 and a multiple shift geartrain 12. The multiple shiftgeartrain 12 is of a hydraulically operative type as is known to theskilled in the art. In this embodiment, the geartrain has four forwardspeed ranges and one reverse speed range and is of the type thatimplements the gear shift by altering a combination of exciting anddeenergizing a solenoid 13 mounted in its hydraulic pressure circuit.The solenoid 13 is controlled by a control unit UAT for shift control.The control unit UAT has pre-stored two kinds of shift characteristicsas shown in FIGS. 4A and 4B and a shift, namely, upshifting anddownshifting, may be performed on the basis of either of the two shiftcharacteristics. The shift characteristic as shown in FIG. 4A is forordinary running, namely, for non-traction control, while the shiftcharacteristic as shown in FIG. 4B is for traction control only. Morespecifically, each of the shift characteristics is set on the basis of avehicle speed and a throttle opening angle as parameters. The shiftcharacteristic as shown in FIG. 4B has no shift line between first andsecond speed stages and further offsets its all shift stages towardlow-speed side as a whole, compared with the shift characteristic asshown in FIG. 4A, thus preventing the torque transmitted to the drivenwheels from increasing.

On top of inputs of a signal for a throttle opening angle from a sensor61 and a signal for a vehicle speed from a sensor 62 (in thisembodiment, a signal for a number of revolutions of the propeller shaft4). the control unit UAT receives output from a control unit UTR fortraction control as will be described hereinafter. It is to be notedthat the shift characteristic as shown in FIG. 4B is selected toimplement shift control only when a signal representing the the tractioncontrol is implemented is inputted from the control unit UTR. Otherwise,the shift control is implemented on the basis of the shiftcharacteristics of FIG. 4A.

Adjustment of Braking Liquid Pressures

As shown in FIG. 1, the right-hand front wheel 1FR is mounted with abrake 21FR having a caliper (a wheel cylinder) 22FR which, in turn, isconnected through a passage 23FR to a liquid pressure adjustment unit 24independently from the others. Likewise, a brake 21FL mounted to theleft-hand front wheel 1FL has a caliper (wheel cylinder) 22FL connectedthrough a passage 23FL to -he liquid pressure adjustment unit 24separately from the others; a brake 21RR mounted to the right-hand rearwheel 1RR has a caliper (wheel cylinder) 22RR which, in turn, isconnected through a passage 23RR to the liquid pressure adjustment unit24; and a brake 21RL mounted to the left-hand front wheel 1RL has acaliper (wheel cylinder) 22RL connected through a passage 23RL to theliquid pressure adjustment unit 24. To the liquid pressure adjustmentunit 24 is fed liquid pressure from a master cylinder 26 as a source forgenerating liquid pressures by stepping down a brake pedal 25 through aliquid pressure passage 27. Further, the liquid pressure adjustment unit24 is fed through a liquid pressure passage 30 with a pressure createdby the liquid pumped up by a pump 28, while it releases a liquidpressure to a reservoir tank 32 through a liquid pressure passage 31.

The liquid pressure adjustment unit 24 has two solenoid valves for eachof the liquid pressure passages 23FL, 23FR, 23RL and 23RR. When theliquid pressure generated by the master cylinder 26 is transmitted toeach of the brakes 21FL, 21FR, 21RL and 21RR as it is, this transmissionis implemented in the same manner as an ordinary braking operation. Andthe braking liquid pressures to the brakes 21FL, 21FR, 21RL and 21RR canbe increased or decreased separately and independently from each other.Control of increasing or decreasing the braking liquid pressures may beimplemented by means of a control unit UABS for an anti-brake system(ABS).

Into the ABS control unit UABS are inputted signals from sensors 63, 64,65 and 66 for sensing speeds of revolutions of the wheels 1FL, 1FR, 1RLand 1RR, respectively. The ABS control unit UABS basically functions toreduce the braking liquid pressure to the brake or brakes of therespective wheel or wheels that is or are locked, when it is sensed thatthe wheel or wheels 1FL, 1FR, 1RL, 1RR are locked for anti-brake lockcontrol, on the one hand, and to increase the braking liquid pressurethereto when the corresponding wheel or wheels is or are returned to anunlock state. During the traction control by the control unit UTR, theABS control unit UABS further functions to conveniently supply brakingliquid pressure only to the brakes 21RL and 21RR for the driven rearwheels 1RL and 1RR, respectively, in response to the signal from thecontrol unit UTR.

Adjustment of Torque Generated by Engine

Referring to FIG. 1, the control unit UTR for traction control brakesthe driven rear wheels 1RL and 1RR through the ABS control unit UABS toreduce the torque applied to the driven rear wheels 1RL and 1RR whilereducing the torque to be generated by the engine. At this end, athrottle opening angle adjustment mechanism 44 is disposed in amechanism for coupling a throttle valve 42 mounted in an air intakepassage 41 for the engine with an accelerator pedal 43.

Referring now to FIG. 2, the throttle opening angle adjustment mechanism44 is shown to comprise three levers, i.e., first lever 112, secondlever 113, and third lever 114, each of which is slidable in theleft-hand and right-hand directions in the drawing. The first lever 112is connected to the accelerator pedal 43 through an accelerator wire112a and the second lever 113 is connected to the throttle valve 42through a throttle wire 112t. The second lever 113 is arranged to beurged by a return spring 121 in the right direction in the drawing,namely, in a direction in which the throttle valve 42 is closed.

The third lever 114 comprises a first engagement section 114a engageablewith the first lever 112 from the right direction in the drawing and asecond engagement section 114b engageable with the second lever 113 fromthe right direction in the drawing. Between the first lever 112 and thethird lever 114 is mounted a first spring 116 so as to urge the firstengagement section 114a of the third lever 114 in a direction in whichthe first engagement section 114a is brought into abutment with thefirst lever 112. Between the second lever 113 and the third lever 114 ismounted a second spring 122 so as to urge the second engagement section114b thereof in a direction to allow the second engagement section 114bto come into abutment with the second lever 113. The urging force of thefirst spring 116 is set to be larger than the urging force of the secondspring 122 and the return spring 121.

The first lever 112 is provided with an engagement section 112b in therightward position in the drawing, thereby regulating the second lever113 from displacing in a predetermined distance toward the rightwarddirection relative to the first lever 112. A press lever 111 is disposedon the leftward side of the third lever 114 in the drawing, which isdriven in the leftward and rightward directions in the drawing by meansof a motor 106 and whose leftward direction beyond a predetermineddistance is blocked by a stopper 123 which is arranged so as to abutwith the press lever 111.

The action of the throttle opening angle adjustment mechanism 44 willnow be described hereinafter. First, when the press lever 111 is inabutment with the stopper 123, no exterior force is applied to the thirdlever 114 so that the first lever 112 is in such a state that the firstlever 112 is always integrally with the second lever 113 and the thirdlever 114, as shown in FIGS. 2(a) and 2(b), thus providing a throttleopening angle in accordance with an accelerator opening angle. In otherwords, the throttle valve is opened at opening angles ranging from 0% to100% as the accelerator opening angle ranges from 0% to 100%. FIG. 2(a)represents the throttle opening angle of 0% while the acceleratoropening angle is 0%. FIG. 2(b) represents accelerator opening angle. Asshown in FIGS. 2(a) and 2(b), there is provided a clearance between thepress lever 111 and the third lever 114, a clearance being set so as tovary from 0% to 100% with respect to the full length of the clearance inaccordance with the corresponding accelerator opening angle varying from0% to 100%. In other words, for example, when the throttle opening angleis 75%, as shown in FIG. 2(b), the clearance accordingly is shortened by75%--in other words, there is still remained a clearance accounting for75% to 100% with the full length therebetween. This can be said ofwhichever opening angle is. Further, it is to be noted that, when thethrottle opening angle reaches 100%, namely, when the acceleratoropening angle reaches 100%, there is no clearance between the presslever 111 and the third lever 114--in other words, the press lever 111being in slight abutment with the third lever 114. When the motor 106 isthen operated to drive the press lever 111 from the state as shown inFIG. 2(b) toward the right in the drawing through the position in whichthe press lever 111 is brought into slight abutment with the third lever114 to the position in which the press lever 111 is further driventoward the right in the drawing to force the third lever 114 to movetoward the right, too, in resistance to the first spring 116., as shownin FIG. 2(c). This operation allows the throttle opening angle to bereturned to the direction of closure even if the accelerator openingangle is the same as previously. FIG. 2(c) represents the state in whichthe throttle opening angle is returned to a full closed state when theaccelerator opening angle is 75%. In this case, the engagement section112b is brought into abutment with the second lever 113. When theaccelerator opening angle is forced to account for 100% as shown in FIG.2(d) from the state as shown in FIG. 2(c), then the first lever 112 isforced to move toward the left in the drawing while the engagementsection 112b forces to move the second lever 113 toward the left, too,in accompany with the leftward movement of the first lever 112. Thischanges the state as shown in FIG. 2(c) in which the throttle openingangle is 0% to the state as shown in FIG. 2(d) in which the throttleopening angle is 25%. In this embodiment, the full opening operation ofthe accelerator pedal can allow the throttle valve 42 to be opened tothe angle accounting for at least 25%. Thus, even if the press lever 111would have been stuck in the state as shown in FIG. 2(c), the automobilecould be driven by itself at the least to a tune-up factory or any otherappropriate locations nearby.

Outline of Traction Control

The control unit UTR for traction control implements brake control viathe ABS control unit UABS, engine control to be implemented bycontrolling the motor 106 for the throttle opening angle adjustmentmechanism 44, and shift control via the control unit UAT for shiftcontrol. The traction control unit UTR is provided with inputs ofsignals through the ABS control unit UABS from the sensors 63, 64, 65and 66 for sensing each of the wheel speeds, a signal of the throttleopening angle from the sensor 67, a signal of the accelerator openingangle from the sensor 68, and a signal of an opening angle of the motor106 from the sensor 69.

FIG. 3 indicates contents of the traction control with a focus on theengine control and the brake control. In FIG. 3, a undriven-wheel speed(sometimes referred to as a vehicle speed by an arithmetic mean value ofspeeds of the revolutions of the left-hand and left-hand front wheels)is indicated by WFN, a spin-judging value as a first judgment value byWFN+10, a spin convergence-judging value as a second judgment value byWFN+3, a target slip value for the engine by SET (WFN+ΔE), and a targetslip value for the brake by SBT (WFN+ΔB). In FIG. 3, the opening angleof the motor 106 is such that the position indicated in FIG. 2(a)corresponds to the motor opening angle of 100% and the positionindicated in FIG. 2(c) corresponds to the the motor opening angle of 0%.The spin-judging value as the first judgment value is set to be thelargest, the target slip value for the brake is set to be next largest,and the target slip value for the engine is smaller than the target slipvalue for the brake. The spin convergence-judging value as the secondjudgment value is set to be the smallest. Given the foregoing, theaccelerator is full open at the time t0, namely, both the throttleopening angle and the motor opening angle are 100%. At this time, theslip value of the driven wheels exceeds the spin-judging value (WFN+10)so that the throttle opening angle (motor opening angle) is reducedimmediately to point SM in order to converge the large slip of thedriven wheel while the braking liquid pressure starts increasing. At thetime t1 which elapsed from the time t0 by a predetermined time period(for example, 0.5 seconds), the slip value of the driven wheel is stilllarger than the convergence-judging value (WFN+3) so that the throttleopening angle is further reduced gradually. While the throttle openingangle has gradually been reduced, the slip value of the driven wheels isreduced below the target slip value SBT for the brake at the time t2, sothat the braking liquied pressure is reduced to almost zero. At the timet3, the slip value of the driven wheels reaches the convergence-judgingvalue (WFN+3). At the time t3, the throttle opening angle is increasedimmediately to the recovery opening angle FTAG in order to avoid adeceleration feeling caused by a reduction in torque to be applied tothe driven wheel (this operation will also be referred to as a fourthcontrol means). During a time period to time t4 thereafter, a feedbackcontrol is implemented so as to cause the slip value of the driven wheelto be reduced to the target slip value SET for the engine. During thefeedback control, the accelerator opening angle is reduced to a greatextent so as to coincide with the throttle opening angle and the motoropening angle at the time t4. Thereafter, the accelerator opening angleis further reduced to a full closed state and, at the same time, thethrottle opening angle is reduced to zero in accompany with a decreasein the accelerator opening angle. On the other hand, the motor openingangle is increased to a full open state. In this state, the press lever111 is brought into abutment with the stopper 123.

Detail of Traction Control

Detail of the traction control will be described hereinafter withreference to the flow charts of FIGS. 8 to 17.

Main Routine (FIG. 8)

After initialization of the system at step P1, it is judged at step P2whether it is a timing for predetermined measurements. When the timingwas confirmed, then data are inputted from the sensors 62 to 69,inclusive, at step P3. Then at step P4, whether a spin has occurred isjudged. This judgment is carried out to position actual spin values ofthe left-hand driven wheel 1RL and the right-hand driven wheel 1RR withrespect to the spin-judging value and the spin convergence-judgingvalue, as shown in FIG. 3. The flow then proceeds to step P5. At stepP5, it is judged whether the road on which the vehicle is currentlyrunning has a split road surface. In other words, it is judged whetherthe vehicle is currently running on the split road surface which differsto a great extent between a friction coefficient μ of the road surfacewith which the left-hand driven wheel 1RL is in contact and a frictioncoefficient of the road surface with which the right-hand driven wheel1RR is in contact.

Thereafter, at step P6, it is judged whether the traction control shouldbe started or ended. At step P7, there is specified the frictioncoefficient μ of the road surface on which the vehicle is currentlyrunning. Then the flow advances to step P8 where it is judged whetherthe traction control is currently proceeding. If the traction control isnow in the process, then it is judged at step P9 whether it is a timingfor implementing the engine control. When it is judged at step P9 thatit is the timing for the engine control, then the flow proceeds to stepP10 wherein an engine control amount is determined, namely, wherein atarget position of the motor 106 is determined which indicates amagnitude of the throttle opening angle. Then at step P11, it isconfirmed whether it is a timing for implementing the brake control.When the timing for the brake control is confirmed, a brake controlamount is determined at step P12. In other words, at step P12, there isdetermined the magnitude of the braking force to be applied to theleft-hand driven rear wheel 1RL or the right-hand driven rear wheel 1RR.The control amounts determined at steps P10 and P12 are then outputtedat step P13 and step P14, respectively. When it is decided at step P8that the traction control is not currently in the process, the flowproceeds to step P16 where the target position of the motor is set to100%, namely, where the mechanism 44 is brought into the state as shownin FIG. 2(a). Then the flow advances to step P13 where the enginecontrol amount is generated. At step P9, when it is confirmed that it isnot the timing for controlling the engine, the flow proceeds to step P11without passage through step P10.

After step P14, shift control is implemented at step P15. Morespecifically, the control unit UAT for the shift control is providedwith instruction signals as to whether the shift characteristic forordinary running as shown in FIG. 4A should be selected or whether theshift characteristic for the traction control as shown in FIG. 4B shouldbe selected.

FIG. 9 (Step P4 of FIG. 8)

At step P21, an undriven-wheel speed (vehicle speed) WFN for thetraction control is set by an arithmetic mean value of the speed ofrevolutions of the left-hand driven wheel, WFL, and the speed ofrevolutions of the right-hand driven wheels, WFR. i.e., ##EQU1##

After step P21, it is judged at steps P22 and P23, respectively, whetherthe speed of revolutions of the left-hand driven wheel, WFL, and thespeed of revolutions of the right-hand driven wheels, WFR, are largerthan the spin-judging value (corresponding to WFN+10 km/hour). When itis judged that either of the speeds of revolutions WFL or WFR is largerthan the spin-judging value at step P22 or P23, then the flow proceedsto step P24 where it is judged whether a time flag is zero. If the thetime flag is zero, the timer value is reset to zero at step P25 and thetimer flag is set to 1 at step P26. Further, at step P27, a spin flag isset to 1 in order to indicate that a slip has occurred which is largerthan the spin-judging value (WFN+10). The processing at steps P24, P25,and P26 is executed for pre-processing to count a setting time from thetime when the slip value of the driven wheel has reached thespin-judging value (WFN+10).

When it is confirmed at step P24 that the timer flag is not zero, thenthe flow proceeds to step P28 where the timer is counted up. Then atstep P29, it is judged whether a timer count value is larger than 500msec. When it is judged as NO, then the flow proceeds to step P27 as itis, on the one hand. When it is judged at step P29 that the timer countvalue is larger than 500 msec, a time lapse flag is set to 1 at step P30in order to indicate that a predetermined time period (for example, 500msec) has elapsed from the time t0 as shown in FIG. 3. Then the flowproceeds to step P27.

When it is judged NO at both of steps P22 and P23, then the flowproceeds to step P31 and it is judged whether the speed of revolutionsof the left-hand driven wheel 1RL, WFL, is smaller than the spinconvergence-judging value (corresponding to WFN+3 km/hour). If YES, thenat step P33, the spin flag is reset to zero. If NO at step P31, then theflow advances to step P32, it is further judged whether the speed ofrevolutions of the right-hand driven wheel 1RR, WFR, is smaller than thespin convergence-judging value (corresponding to WFN+3 km/hour). If YES,the spin flag is reset to zero at step P33.

When it is judged NO at step P32 or after the resetting to zero at stepP33, the time lapse flag is reset to zero at step P34 and the timer flagis reset to zero at step P35.

After step P35 or step P27, it is judged at step P36 whether it is thetime that is immediately after the spin has occurred, in other words,immediately after the time t0 as shown in FIG. 3 has elapsed. When thedecision at step P36 is YES, flag JF is set to 1 at step P38. If thedecision at step P36 is NO, then it is judged at step P37 whether it isimmediately after the spin has been converged, namely, after it is justafter the time t3 as shown in FIG. 3. If YES, then flag JF is set to 2at step P40. If NO in the decision at step P37, flag JF is reset tozero.

FIG. 10 (Step P5 of FIG. 8)

First, it is judged at step P51 whether the speed of revolutions of theleft-hand driven wheel, WRL, is larger than WFN+2. When the decision atstep P51 is NO, it is further judged at step P52 whether the revolutionspeed of the left-hand driven wheel, WRL, is larger than WFN+1.5. If YESat step P52, then the flow advances to step P53 and flag SPL for theleft-hand driven wheel is reset to 0. If YES in the decision at stepP51, then flag SPL for the left-hand driven wheel is set to 1 at stepP54.

Likewise, for the right-hand driven wheel 1RR, it is judged at steps P55and P56, respectively, whether its speed of revolutions of theright-hand driven wheel, WRR, is larger than WFN+2 and smaller thanWFN+1.5, and flag SPR is set or reset at steps P57 and P58 in the samemanner as for the left-hand driven wheel 1RL.

Then at step P59, it is judged whether the flag SPL differs from theflag SPR, in other words, whether one of them is set to 1 and the otheris reset to 0. When it is judged at step P59 that the flag SPL differsfrom the flag SPR, it is decided that the road on which the vehicle iscurrently running is a split road surface so that split flag is set to 1at step P61. When it is judged at step 59 that the flag SPL is the sameas the flag SPR, in other words, when the flag SPL and the flag SPR areboth 1 or 0, it is decided that the vehicle is currently on a non-splitroad so that split flag is reset to zero at step P60.

FIG. 11 (Step P6 of FIG. 8)

At step P71, it is judged whether the accelerator opening angle issubstantially full closed (for example, smaller than 5%). If YES, it isdecided that no traction control is required so that traction flag isreset to zero at step P76.

When it is judged at step P71 that the accelerator opening angle is notalmost full closed, it is then judged at step P72 whether the tractionflag is set to 1. If YES, namely, if it is decided that the tractioncontrol is in the process, then the flow proceeds to step P75 and it isjudged whether the accelerator opening angle (which is the same as thethrottle opening angle to be determined by the accelerator openingangle) is smaller than a current motor opening angle (which is the sameas the throttle opening angle to be determined by the position of themotor 106). If YES, the flow proceeds to step P76 and the tractioncontrol is suspended by resetting the traction flag to zero. If NO atstep P75, then control is ended as it is.

When it is decided at step P72 that the traction control is notcurrently active, it is then judged at step P73 whether the spin flag isset to 1 (see steps P27 and P33 in FIG. 9). When the decision at stepP73 is in the affirmative, the traction flag is set to 1 at step P74 inorder to execute the traction control. If NO at step P73, the controlends as it is.

As have been described hereinabove, in this embodiment, the condition ofstarting the traction control is set to be an occurrence of the slipthat is larger than the spin-judging value (WFN+10), on the one hand,and the condition of suspending the traction control is set to be asubstantially full closure of the accelerator opening angle or adecrease of the accelerator opening angle to such a small level that notraction control is required (by the judgment at step P75).

FIG. 12 (Step P7 of FIG. 8)

In FIG. 12, assumption of the road friction coefficient μ is determinedon the basis of acceleration G of the vehicle body and the vehiclespeed. It is further noted that no detection of the vehicle bodyacceleration is made for a while after the time t3 of FIG. 3 because thetorque applied to the driven wheels is reduced to a significantly largeextent and no sufficient degree of the acceleration can be obtainedduring that period. This prevents the acceleration from reflecting uponFTAG (FIG. 3) for a while after the time t3 and avoids the situation inwhich an acceleration feeling is impaired.

Given the foregoing, it is judged at step P81 whether the spin flag isset to 1. If YES in the decision at step P81, the spin timer is reset tozero at step P92 and the acceleration of the vehicle body, Gn, iscalculated by subtracting the latest vehicle speed (one control cycleago), WFNn-1, from the current vehicle speed, WFNn, and then multiplyingthe difference by a predetermined correction coefficient, Gk.Thereafter, at step P89, it is judged whether the maximum accelerationof the vehicle body, Gmax, stored so far, is smaller than the vehiclebody acceleration, Gn, calculated at step P88. If it is judged that Gmaxis smaller than Gn, on the one hand, then the flow proceeds to step P90and the current vehicle body acceleration Gn is updated to a new maximumacceleration, Gmax and the flow proceeds to step P91. If it is decidedat step P89 that the current vehicle body acceleration Gn is larger thanthe latest maximum acceleration Gmax, then the flow proceeds to step P91as it is without passage through step P90.

At step P91, the road friction coefficient is assumed from the map asshown in FIG. 5 on the basis of the vehicle body acceleration Gmax andthe current vehicle speed WFN. As shown in FIG. 5, the road frictioncoefficient μ is classified into five stages such that the larger thestage the larger the friction coefficient μ is. The same thing can besaid of FIGS. 6 and 7 as will be described hereinafter.

After step P91, the current spin flag is changed as the latest spin flagat step P85.

At step P81, when it is decided that the spin flag is not set to 1, thenthe flow proceeds to step P82 and it is further judged whether thelatest or previous spin flag is set to 1. If YES, the spin timer is setat step P83 to a predetermined value A (for example, 0.2 to 0.3 second)and then the latest spin flag is reset to zero at step P84. Thereafter,the process of step P85 is implemented.

When it is decided at step P82 that the previous spin flag is not set to1, the flow proceeds to step P86 where it is judged whether the spintimer is set to zero When the decision at step P86 is NO, the spin timeris counted down at step P87 and then the flow advances to step P85 wherethe same process as described hereinabove is implemented. If thedecision at step P86 is YES, then the flow proceeds to step P88 and theprocess is repeated in the same manner as described hereinabove.

FIG. 13 (Step P10 of FIG. 8)

As shown in FIG. 13, the position of the motor 106, namely, the throttleopening angle, is determined. In this case, there are also determinedthe amount of torque rapidly decreased (the setting of SM) at the timet0 as shown in FIG. 3, the recovery opening angle (FTAG) at the time t3as shown in FIG. 3, and the amount of torque to be gradually decreasedwhen the spin value does not reach the spin convergence-judging value(WFN+3) within a predetermined time period from the time t0 as shown inFIG. 3. In this embodiment, even if a large extent of the slip wouldoccur during the traction control, which exceeds the spin-judging value,it is provided that the torque to be applied is not decreased rapidly atthe time tO and decreased gradually after the time t2 as shown in FIG.3, and increased temporarily at the time t3 as shown in FIG. 3.

Given the foregoing, as shown in FIG. 13, at step Q1, the lower throttlelimit value SM at the time t0, as shown in FIG. 3, is determined byreferring the assumed friction coefficient μ and the current vehiclespeed WFN to the map as shown in FIG. 6. The map as shown in FIG. 6 isset as the minimum value that tis required to the least possible extentin order to maintain the current vehicle speed. Then at step Q2,recovery opening angle FTAG at the time t3 is determined by referringthe assumed friction coefficient μ and the current vehicle speed WFN tothe map as shown in FIG. 7.

At step Q3, it is then judged whether flag JF is currently set to 1(steps P38 to P40 in FIG. 9). When it is decided at step Q3 that theflag JF is set to 1, the flow advances to step Q4 and it is then judgedwhether the spin value has reached the spin-judging value (WFN+10) forthe first time, namely, whether the slip having the magnitude exceedingthe spin-judging value has occurred during non-traction control. If thedecision at step Q4 is YES, the target position (target opening angle)of the motor 106, MTAGn, is set at step Q5 as a value corresponding tothe lower limit value SM determined at step Q1. Then at step Q6, thecurrent lower limit value SM is updated to SM1 and the flow proceeds tostep Q20.

When it is decided at step Q3 that the flag JF is not set to 1, it isthen judged at step Q7 whether the flag JF is set to 2. If YES, then itis further judged at step Q11 whether the spin value has been judged forthe first time during the traction control. If the decision at step Q11is in the affirmative, the target opening angle MTAGn of the motor 106is set to the recovery opening angle FTAG (FIG. 3) and then the flowproceeds to step Q20.

When it is decided NO at step Q7, the flow proceeds to step Q8 and it isthen judged whether the time lapse flag is set to zero (steps P30 andP34 in FIG. 9). It is to be noted that the decision is in the negativeat step Q8 means that the slip value of the driven wheel is notdecreased to a sufficient extent up to the spin convergence-judgingvalue within the predetermined time period from the time t0 as shown inFIG. 3. At this time, the product obtained by multiplying SM1 with 0.9and the product is updated to SM1 at step Q9. Then at step Q10, theupdated SM1 is set to SM and the flow proceeds to step Q5. Transfer tostep Q5 decreases gradually the throttle opening angle (the lower limitSM being decreased by 10 percents per every control cycle) during a timeperiod when the slip value of the driven wheel is going down to the spinconvergence-judging value (WFN+3).

When it is decided at step Q8 that the time lapse flag is set to zero,then the flow transfer to step Q13 where it is judged whether the splitflag is in a 1 state (see steps P60 and P61 of FIG. 10). If the decisionat step Q13 is in the negative, it means that the road on which thevehicle is currently running is not a split one. At this time, whicheverlarger, the speeds of revolutions of the left-hand or right-hand drivenwheels, WRL or WRR, is set as a wheel speed SE to be subjected to enginecontrol. If YES at step Q13, it means that the road on which the vehicleis running is a split road and, in this case, whichever smaller, thespeeds of revolutions of the left-hand or right-hand driven wheels, WRLor WRR, is set as a wheel speed SE to be subjected to engine control. Asdescribed hereinabove, it is noted that, when the vehicle is running ona non-split road, on the one hand, the wheel speed subjected to controlis selected with importance attached to stability and that, when it isrunning on a split road, on the other hand, the wheel speed subjected tocontrol is selected with importance attached to acceleration. Insummary, the vehicle is attempted to run with effective utilization of agrip of the driven wheel that is less likely to slip.

After step Q14 or step Q15, a slip-addition value ΔE (3<ΔE<10) isdetermined in accordance with the road surface friction coefficient μ atstep Q16. At step Q17, a target slip value SET for the engine is thencalculated by adding the slip-addition value ΔE to the current vehiclespeed (undriven wheel speed) WFN (see FIG. 3).

Then at step Q18, a feedback control amount (a throttle-varying value)ΔM is determined so as to reach the current speed of revolutions of thedriven wheel, SE, to the target slip value SET for the engine. Thefeedback control amount ΔM is calculated by the following equation (1):

    ΔM=KP×(EN.sub.n -EN.sub.n-1)+KI×EN.sub.n (1)

where

KP: proportional coefficient;

KI: integral coefficient;

EN: SET-SE

n: suffix

Then flow proceeds to step Q19 and a target current motor position MTAGnis calculated by adding the feedback control amount ΔM to the latesttarget motor position MTAG_(n-1). Thus, MTAGn=MTAG_(n-1) +ΔM.

After step Q19 or after steps Q6 or Q12, the flow is transferred to stepQ20 where the target motor position MTAGn is subjected to limitprocessing so as to be positioned between the lower limit value SM andthe upper limit value 100%.

FIG. 14 (Step P13 of FIG. 8)

The flowchart as shown in FIG. 14 is to set an optimal response speed inorder to allow the motor to reach a predetermined target position MTAGnand to realize (output) this target position MTAGn. The response speedis set so as to prevent occurrence of a shock to be caused due toabutment of the press lever 111 with the third lever 114 by driving themotor 106 and reduction of an acceleration feeling due to changes inurging force by the urging means 116.

First, at step Q31, it is judged whether the target motor position MTAGnaccounts for 100%, in other words, whether no traction control isrequired. If YES, it is further judged at step Q32 whether the currentposition (opening angle) of the motor 106 is larger than a predeterminedvalue αm (for example, 80%). If the decision at step Q32 is in thenegative, the flow proceeds to step Q40 where the motor position MTAGnis set as a final target motor position MTAGF as it is, and the finaltarget motor position MTAGF is generated at step Q39.

If the decision at step Q32 is in the affirmative, a filter timeconstant D is set as Dm at step Q33 and the flow proceeds to step Q38.

When it is decided NO at step Q31, it is then judged at step Q34 whethera current accelerator opening angle, ACP, is smaller than the targetmotor position MTAGn. If YES, then at step Q41, it is judged whether thedifference, αa obtained by subtracting the current accelerator openingangle ACP from the target motor position MTAGn, is smaller than apredetermined value (for example, 5%). If MTAGn-ACP<αa, on the one hand,the filter time constant D is set as Da at step Q37 and then at stepQ38. If MTAGn-ACP≧αa at step Q41, the flow is transferred to step Q40.

If ACP≧MTAGn at step Q34, it is further judged at step Q35 whether thecurrent accelerator opening angle, AC P, is smaller than a predeterminedvalue αc (for example, 5%). If ACP<αc, on the one hand, the filter timeconstant D is set as Dc at step Q36 and then the flow proceeds to stepQ38. If ACP≧αc, on the other hand, the flow is transferred to step Q40.

At step Q38, the final target motor position MTAGF of the motor 106 issubjected to filter processing on the basis of the following equation(2), which corresponds to a variation in the motor 106 per controlcycle, i.e., in the throttle opening angle).

    MTAGF=D×MTAGn+(1-D)×MTAG.sub.n-1               (2)

After step Q38, the final target motor position MTAGF is outputted atstep Q39.

It is to be noted herein that, in this embodiment, the filter timeconstant is set as follows:

    Dm<Dc<Da<1

FIG. 15 (Step P12 of FIG. 8)

First, at step Q51, the slip-addition value ΔB is determined inaccordance with the road surface friction coefficient μ and, at stepQ52, the target slip value SBT for the brake is calculated by adding theslip-addition value ΔB to the vehicle speed (undriven-wheel speed) WFN(FIG. 3). In this embodiment, it is to be noted herein that theslip-addition value ΔB is set to be larger than the other slip-additionvalue ΔE, as shown in FIG. 13 (namelY, SBT>SET), in order to allowtraction control to be implemented as mainly as possible by the engine.

Then at step Q53, feedback control amounts TCCR and TCCL are determinedby PI control so as to allow the current slip values of the right-handand left-hand driven wheels 1RR and 1RL to reach the target slip valueSBT independently from each other. The feedback control amounts TCCR andTCCL are calculated by the following equations (3) and (4),respectively:

    TCCR=K.sub.BP ×(ENR.sub.n -ENR.sub.n-1 +K.sub.BI ×ENR.sub.n (3)

    TCCL=K.sub.BP ×(ENL.sub.n -ENL.sub.n-1 +K.sub.BI ×ENL.sub.n (4)

where

ENR=WRR-SBT;

ENL=WRL-SBT;

K_(BP) =proportional constant;

K_(BI) =integral constant; and

n=suffix

At step Q54, the feedback control amounts TCCR and TCCL are then codedto integers for the outputs to the ABS control unit UABS. In otherwords, TCCR is coded to TCTR and TCCL is coded to TCTL. Then at stepQ55, the coded integers TCTR and TCTL are subjected to limit processingin order to be contained within the range from -7 to +7.

FIG. 16 (Step P14 of FIG. 8)

The flow chart of FIG. 16 corresponds to the system in which thetraction control is selected from the ABS control.

First, at step Q57, it is judged whether the vehicle is running in ahigh speed range where there is no possibility of causing downshifting,more specifically, whether the vehicle speed is, for example, over 125km per hour (FIG. 4B). If the decision is YES, the flag TBR indicatingthe presence or absence of the signal for requiring the brake for thetraction control is set to 1 at step Q58. In other words, the brakecontrol for the traction control is inhibited forcibly by setting TBR to1 at step Q58 (reference is made to description of the ABS control ofFIG. 20).

After step Q58 or when the decision at step Q57 is NO, the flow proceedsto step Q60.

Then at step Q60, the signal TBR is generated to the ABS control unitUABS, which represents whether or not brake control resulting from thetraction control should be required. When TBR=0, on the one hand, thismeans that request for the brake control by the traction control isrequired. When TBR=1, on the other, this means that no request for thebrake control is required.

Then at step Q61, the signal SOT is generated to the ABS control unitUABS. The SOT signals represent an increase or decrease in brakingliquid pressures for the left-hand and right-hand driven wheels 1RL and1RR and contain signals TCTR and TCTL which represent magnitudes ofthose braking liquid pressures. At step Q62, echo-back and SIT signalsare inputted from the ABS control unit UABS.

At step Q63, it is judged whether it is currently in the process of ABScontrol on the basis of SIT signals from the ABS control unit UABS. Whenthe ABS control is currently in the process, ABS flag is set to 1,indicating that the ABS control is currently in the process. If it isdecided at step Q63 that the ABS control is not in the process, then theflog proceeds to step Q65 where the ABS flag is reset to zero.

In usual cases, it cannot be considered that the ABS control is requiredconcurrently with the traction control. For example, however, there maybe occasions where the vehicle has jumped and the driven wheel has beenfallen down violently onto the road surface during running at a highspeed. At this time, the driven wheel could undergo braking action fromthe road surface when fallen thereonto and require the ABS controltemporarily. It is to be noted herein how to take advantage of judgmentresults on bad roads or good roads obtained by utilizing such phenomenawill be omitted herefrom.

FIG. 17 (Step P15 of FIG. 8)

At step Q71, whether the traction control is being implemented isjudged. When it is decided that the traction control is currently in theprocess, on the one hand, the control unit UAT for shift control isprovided at step Q72 with a signal indicating that the shiftcharacteristic for traction control, as shown in FIG. 4B, should beselected. Then at step Q73, flag TE is set to 1 in order to indicatethat the shift characteristic for the traction control has beenselected.

When it is decided at step Q71 that the traction control is not in theprocess, on the other hand, it is then judged whether the flag TE is setto 1 at step Q74. If NO at step P74, the control unit UAT is provided atstep Q76 with an instruction indicating a selection of the shiftcharacteristic for ordinary running as shown in FIG. 4A. Then the flagTE is reset to 0 at step Q77.

If it is decided at step Q74 that the flag TE is set to 1, it is thenjudged at step Q75 whether the accelerator opening angle be zero. Whenthe decision is made that the accelerator opening angle is not zero,then the flow proceeds to step Q72 where the selection of the shiftcharacteristic for traction control as shown in FIG. 4B is instructed.When it is decided at step Q75 that the accelerator opening angle iszero, then the flow advances to step Q76 where the shift characteristicfor ordinary running, as shown in FIG. 4A, is selected.

In this embodiment, as described hereinabove, when the shiftcharacteristic for traction control is once selected, it is returned tothe shift characteristic for ordinary running (means for setting shiftconditions) on condition that the accelerator is full closed. Thisarrangement can prevent reoccurrence of an excessive slip resulting froma change of the shift characteristics.

Relationship of Control Unit UTR with Control Unit UABS

The control unit UTR for traction control and the control unit UABS forABS control are composed of microcomputers. The following is a briefdescription on the signals which are transmitted or received by thecontrol units.

The control unit UTR generates signals indicating presence or absence ofrequirements for traction control as TBR signals, as describedhereinabove. The 1 signal means absence of requirements for tractioncontrol while the 0 signal means presence of requirements for tractioncontrol.

The control units UTR and UABS have communication modules MT and MA,respectively, for serial transmission (FIG. 1). With these modules MTand MA, 8-bit signals are transmitted and received by means oftime-division transmission.

The SOT signal to be transmitted from the control unit UTR is shown inFIG. 18. The signal SOT has 8 bits consisting of from bit b to bit i.The bits b to e are for the left-hand rear wheel 1RL, and the bits b tod indicate a control time period of the braking liquid pressure. Morespecifically, the time period for which the solenoid valve of the liquidpressure adjustment unit 24 is kept turned ON is represented by theinteger from 0 to 7 (see step Q55 of FIG. 15). The bit e indicatesdistinction of an increase or a decrease in the pressure, namely, adistinction between "+" or "-". The bits f to i are for the right-handrear wheel 1RR. The bits f to h indicate a control time period of thebraking liquid pressure and the bit i indicates distinction of anincrease or decrease in the pressure.

The signal SIT to be transmitted from the ABS control unit UABS is shownin FIG. 19 and consists 8 bits represented by symbols b to i. However,substantially, only four of the 8 bits, i.e., bits d, e, h and i, areused. The bit h is an echo-back for confirmation of acknowledgement oftraction control data and indicates an agreement when h=0 or the signalTBR being "1" (high) and a disagreement when h=1. A reverse signal ofthe bit h is the bit d. The bit i indicates the state of the ABScontrol. The state of i=0 indicates a non-ABS control and the state ofi=1 indicates the ABS control being in the process. The bit e is areverse signal of the bit i.

The ABS control unit UABS controls the braking liquid pressure inresponse to output of the signal for adjustment of the braking liquidpressure adjustment by sending data for traction control to the ABScontrol unit UABS when request for the traction is made from the controlunit. In other words, the control unit UTR is selected prior to the ABScontrol unit UABS. On the other hand, it is also possible to permit datatransfer for traction control only when the ABS control unit UABS sendsa request signal (for example, a request signal being generated insynchronization with a timing for output of a signal for braking liquidadjustment during non-ABS control). In other words, the ABS control unitUABS is selected prior to the control unit UTR.

Detail of ABS Control

As is known to the art, the ABS control is to cause no wheel to belocked on pavement due to a too frequent braking. In this embodiment,the state in which each of the wheels is locked is represented by a lockvalue as will be defined by the following equation (5): ##EQU2##

As is apparent from the equation (5) above, the larger lock value S1means the greater tendency of locking the wheel. Generally speaking, thebraking force (braking liquid pressure adjusting unit 24) for each ofthe wheels is subjected to feedback control, for example, PI control, soas for the lock value S1 to reach a target value ranging from 0.1 to0.2. The target value may vary with the friction coefficient, on a roadsurface, or the like. For instance, the target value may be 0.1 for asnow-covered road and 0.2 for dry pavement.

In this embodiment, on the one hand, the condition of starting the ABScontrol is set as the time when the lock value S1 becomes greater than apredetermined start judging value R1 (R1>R2). On the other hand, thecondition of ending the ABS control is set as the time when the lockvalue S1 becomes smaller than a predetermined end judging value R2(0<R2<0.1).

Description will be made of an example of the ABS control by payingattention to the instance where the traction control is selected priorto the ABS control, in conjunction with the flow chart as shown in FIG.20.

First, after the wheel speed of each wheel is read at step Xl, each ofthe wheel speeds is generated to the control unit UTR for the tractioncontrol at step X2. This data may be contained in the SIT signal.

At step X6, the vehicle speed may be presumed at step X6 insubstantially the same way as in part of the above description on thetraction control. Thereafter, at step X7, the lock value S1 iscalculated from the equation (5) above.

After step X7, it is judged at step X8 whether lock flag is set to 1that indicates the ABS control under way. When the decision at step X8is NO and it is decided that the ABS control is not currently under way,the flow proceeds to step X9 where it is then judged whether the currentlock value is greater than the predetermined start judging value R1 forthe ABS control. When it is decided at step X9 that S1>R1, the lock flagis set to 1 at step X10 and a brake control amount is determined, forexample, by the PI control, at step X11. Then at step X12, the brakecontrol amount is generated to the braking liquid pressure adjustingunit 24.

When the decision at step X9 is NO, at step 3, it is judged whether theflag TBR is zero or not. If it is decided that the flag TBR is set tozero--in other words, that the brake control is required for thetraction control--signals TCTR and TCTL indicating brake control amountsfor the respective wheels are read from the control unit UTR at step X4.Reference may also be made to the description of FIG. 18. Thereafter, atstep X5, the brake control amounts read at step X4 are then outputted tothe braking liquid pressure adjusting unit 24.

When it is decided at step X8 that the ABS control is under way, theflow advances to step X13 and it is judged whether the current lockvalue S1 is smaller than the predetermined end judging value R2 for theABS control. If the decision is NO, on the one hand, then the flowproceeds to step X11 and the brake control by the ABS control iscontinued. If the decision at step X13 is that the lock value S1 issmaller than the value R2, on the other, the lock flag is set to zero atstep X14.

After the steps X5, X12 and X14 and when the decision at step X3 is NO,the flow proceeds to step X15, in each case, where the SIT signal isgenerated to the control unit UTR for the traction control. Reference isalso made to description of FIG. 19.

It is to be understood herein that the present invention is notrestricted to those embodiments as described hereinabove, but itcontains various variants and modifications within the spirit and scopeof the invention.

(1) In place of a slip amount based on a deviation between adriven-wheel speed and the vehicle speed, there may be used, as the slipvalue, a ratio of the driven-wheel speed to the vehicle speed or a valueobtained by subtracting the vehicle speed from the driven-wheel speedand then dividing the difference with the driven-wheel speed. Likewise,the lock value can be represented by a deviation between the wheel speedand the vehicle speed.

(2) In decreasing the torque to be applied to the driven wheels, it ispossible to implement the brake control premominantly and the enginecontrol secondarily (SET>SBT). The traction control may be implementedby means of the brake control only.

The present invention may be embodied in other specific forms withoutdeparting from the spirit and scope thereof. The Present embodiments asdescribed hereinabove are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims, and all the changes, modifications andvariations which come within the meaning and range of equivalency of theclaims are therefore intended to be encompassed within the spirit andscope of the invention.

What is claimed is:
 1. A slip control system for a vehicle, comprising:abraking force adjusting means for adjusting a braking force of a brakefor a driven wheel; a slip detecting means for detecting a slip value ofthe driven wheel on a road surface; a brake control means for operatingthe brake by controlling the braking force adjusting means when the slipvalue detected by the slip detecting means reaches a a predeterminedvalue; a vehicle speed detecting means for detecting a vehicle speed;and an inhibition means for inhibiting operation of the brake controlmeans at the time of running at high speed at which the vehicle speeddetected by the vehicle speed detecting means is equal to or greaterthan the predetermined value.
 2. A slip control system as claimed inclaim 1, wherein:a transmission interposed between an engine and thedriven wheel of the vehicle is an automatic transmission capable ofimplementing a shift on the basis of a predetermined shiftcharacteristic; and the predetermined shift characteristic comprises afirst shift characteristic for ordinary running and a second shiftcharacteristic for traction control; wherein shift control of theautomatic transmission is implemented on the basis of the second shiftcharacteristic only when control is implemented by the brake controlmeans.
 3. A slip control system as claimed in claim 1, furthercomprising:an automatic transmission interposed between an engine andthe driven wheel of the vehicle; a shift-characteristic storage meansfor storing the first shift characteristic for ordinary running and thesecond shift characteristic for traction control; a shift-characteristicselecting means for selecting the second shift characteristic fortraction control when control is implemented by the brake control meansand for selecting the first shift characteristic for ordinary runningwhen control is implemented by a control means other than the brakecontrol means; and a shift control means for controlling a shift of theautomatic transmission on the basis of the shift characteristic selectedby the shift-characteristic selecting means.
 4. A slip control system asclaimed in claim 3, wherein:the first shift characteristic for ordinaryrunning is set so as to shift between a lowest speed stage and a highestspeed stage; and the second shift characteristic for traction is set soas not to shift to the lowest speed stage.
 5. A slip control system asclaimed in claim 3, wherein the second shift characteristic is set so asto make its shift line located on a speed side lower than the firstshift characteristic.
 6. A slip control system as claimed in claim 3,further comprising a switching-condition setting means for setting acondition as a switching condition when the shift-characteristicselecting means selects the first shift characteristic from a statewhere the second shift characteristic is selected, a condition beingsuch that an accelerator opening angle reaches substantially zero.
 7. Aslip control system as claimed in any one of claims 2 to 6, wherein thepredetermined vehicle speed when operation of the brake is inhibited bythe inhibition means is set as a vehicle speed which causes nodownshifting on the basis of the shift characteristic for traction.
 8. Aslip control system as claimed in claim 1, whereina transmissioninterposed between an engine and the driven wheel of the vehicle is anautomatic transmission capable of implementing a shift on the basis of apredetermined shift characteristic; and the predetermined vehicle speedwhen operation of the brake control means is inhibited by the inhibitionmeans is set as a vehicle speed which causes no downshifting on thebasis of the shift characteristic.
 9. A slip control system as claimedin claim 1, further comprising:a torque adjusting means for torque to begenerated by an engine; and an engine control means for reducing torqueto be applied to the driven wheel by controlling the torque adjustingmeans when the slip value detected by the slip detecting means reaches apredetermined value or higher.
 10. A slip control system as claimed inclaim 1, wherein the brake control means subjects the braking forceadjusting means to feedback control so as for the slip value of thedriven wheel detected by the slip detecting means to reach apredetermined target value.
 11. A slip control system as claimed inclaim 1, further comprising:a torque adjusting means for torque to begenerated by an engine; and an engine control means for subjecting thetorque adjusting means to feedback control so as for the slip valuedetected by the slip detecting means to reach a predetermined firsttarget value; and wherein the brake control means subjects the brakingforce adjusting means to feedback control so as for the slip value ofthe driven wheel detected by the slip detecting means to reach apredetermined second target value which is set as a value different fromthe predetermined first target value.
 12. A slip control system asclaimed in claim 11, wherein wherein the first target value is set to avalue smaller than the second target value.
 13. A slip control system asclaimed in claim 11, further comprising a second control means forreducing torque to be applied to the driven wheel temporarily by apredetermined decrement by controlling the torque adjusting means priorto the engine control mean, when the slip value detected by the slipdetecting means reaches a value larger than a first judgment value whichis set larger than both the first target value and the second targetvalue.
 14. A slip control system as claimed in claim 13, furthercomprising a third control means for reducing the torque to be appliedto the driven wheel gradually by controlling the torque adjusting meansprior to the engine control means when the slip value detected by theslip detecting means does not become smaller than a second judgmentvalue which is set as a value smaller than the first judgment value evenif a predetermined period of time elapsed after start of control by thesecond control means.
 15. A slip control system as claimed in claim 14,further comprising a fourth control means for raising the torque to beapplied to the driven wheel by a predetermined increment by controllingthe torque adjusting means prior to the engine control means when theslip value of the driven wheel detected by the slip detecting means isreduced to the second judgment value.
 16. A slip control system asclaimed in claim 13, wherein control by the engine control means startsafter the end of the control by the second control means.
 17. A slipcontrol system as claimed in any one of claims 13 to 16, wherein thefirst target value is set as a value which is smaller than the secondtarget value.
 18. A slip control system as claimed in claim 17, whereinthe second judgment value is set as a value which is smaller than thesecond target value.
 19. A slip control system as claimed in claim 1,further comprising an ABS control means for implementing an anti-lockcontrol for preventing the wheel from locking with respect to a roadsurface by controlling the braking force adjusting means when thebraking force is applied to the wheel by manual operation;whereincontrol of the braking force adjusting means by the brake control meansis implemented through the ABS control means.
 20. A slip control systemas claimed in claim 19, wherein the braking force adjusting means iscontrolled in accordance with a signal for requiring braking prior tothe anti-lock control when the signal for requiring braking istransferred from the brake control means to the ABS control means.
 21. Aslip control system as claimed in claim 19, wherein the ABS controlmeans controls the braking force adjusting means on the basis forrequiring braking from the brake control means only when no anti-lockcontrol is implemented.
 22. A slip control system as claimed in claim14, wherein:control by the second control means is implemented onlyprior to start of control by the engine control means and control by thethird control means is implemented only prior to start of control by theengine control means; and wherein control by the second and thirdcontrol means is inhibited during control by the engine control means.23. A slip control system as claimed in claim 13, wherein a controlamount by the second control means is set on the basis of a frictioncoefficient μ on a road surface.
 24. A slip control system as claimed inclaim 15, wherein a control amount by the fourth control means is set onthe basis of a friction coefficient μ on a road surface.
 25. A slipcontrol system as claimed in claim 1, wherein either of front wheels orrear wheels is driven wheels and the other is undriven wheels; andtheslip detecting means calculates the slip value of the driven wheel onthe basis of a speed of revolutions of the undriven wheel and a speed ofrevolutions of the driven wheel.
 26. A slip control system as claimed inclaim 19, further comprising a lock value detecting means for detectinga lock value of the driven wheel with respect to a road surface;whereinthe ABS control means subjects the braking force adjusting means tofeedback control so as for the lock value of the driven wheel detectedby the lock value detecting means to reach a predetermined target value.27. A slip control system as claimed in claim 26, wherein either offront wheels or rear wheels is driven wheels and the other is undrivenwheels; andthe lock value detecting means calculates the lock value ofthe driven wheel on the basis of a speed of revolutions of the undrivenwheel and a speed of revolutions of the driven wheel.